There has recently been active development of electronic devices (compound semiconductor devices) in which a GaN layer as an electron transit layer and an AlGaN layer are formed over a substrate. One of such compound semiconductor devices is a GaN high electron mobility transistor (HEMT). The use of a GaN HEMT as a switch for voltage source inverter enables both reduction of on-resistance and increase of withstand voltage. Reduction of standby power consumption and increase of operating frequency are also possible, as compared with Si transistors. For these reasons, reduction of switching loss and power consumption for inverters is possible. In addition, a reduction in size is possible as compared with Si transistors having a similar performance.
In a GaN HEMT, in which a GaN layer is used as an electron transit layer and AlGaN as an electron supply layer, strain is produced on the AlGaN due to the difference in lattice constant between AlGaN and GaN. For this reason, piezoelectric polarization occurs and a highly concentrated 2-dimensional electron gas (2DEG) is obtained. This GaN HEMT is therefore applied to high output power devices.
It is however difficult to manufacture a GaN substrate having good crystallinity. GaN layers and AlGaN layers are therefore conventionally formed over a Si substrate, a sapphire substrate or a SiC substrate by heteroepitaxial growth. In particular, Si substrates of a large diameter and a high quality are easily available at low costs. There has therefore been an increase in research into a structure where the GaN layer and AlGaN layer are grown over a Si substrate.
There is however a large difference in thermal expansion coefficient between the GaN layer/the AlGaN layer and the Si substrate. On the other hand, a high temperature treatment is required for epitaxial growth of GaN layer and AlGaN layer. Warping or cracking of the Si substrate may therefore occur due to the difference in thermal expansion coefficient during such high temperature treatment. The problem arising from the difference in thermal expansion coefficient can be suppressed by forming a buffer layer having a superlattice structure, in which two compound semiconductor layers having a different composition are alternately stacked, between the GaN layer and the AlGaN layer.
However, in conventional compound semiconductor devices utilizing an superlattice structure as a buffer layer, it is difficult to obtain good crystallinity of the electron transit layer and the electron supply layer formed thereover. Examples of the related art are Japanese Unexamined Patent Application Publications 2007-258230 and 2010-245504.